Mitochondrial DNA mutations, energy metabolism and apoptosis in aging muscle.

Locomotor functional decline and loss in muscle mass with age is virtually a universal characteristic that has been documented in several species, including worms, fruit flies, rodents, non-human primates and humans. The age-related loss of muscle mass and strength (sarcopenia) represents an important risk factor for disability and mortality in older subjects and has been linked with cellular energy deficit and increased apoptosis at old age. Many key theories on aging describing the mechanisms underlying sarcopenia are now focused on the mitochondria because of their dichotomous role in controlling life and death processes within myocytes. Mitochondria represent the main producers of cellular energy in the form of adenosine triphosphate, but are also considered a key regulatory center of apoptosis. Unknown factors leading to a decrease in aerobic energy efficiency are linked with mitochondrial mutations which may result into apoptosis. Moreover, deregulation of autophagy (degradation and recycling of long-lived protein and organelles, such as the mitochondria) in post-mitotic tissue might also be responsible for the age-associated cellular energy failure. Alterations in specific signaling pathways, such as AMP-activated protein kinases, play a role in both cell survival response and apoptotic response depending on energy depletion. Evidence supports that apoptosis occurring in aging skeletal muscle may be due, in part, to the progressive decline in mitochondrial function and the resulting energy depletion within the cell. In turn, mitochondrial dysfunction is partly due to the accumulation of oxidative damage to macromolecules, including mitochondrial DNA, RNA and proteins, essential components for optimal functioning of mitochondria. Evidence concerning these series of events leading to energy depletion and apoptosis are discussed.